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MYTILUS. 147 
Britannica’ (not that of his Supplement, which is the Crenella 
nigra of the ‘ British Mollusca,’) and the Modiola levigata of 
Gray, &c. Both are nearly allied, and both would undoubtedly 
have been considered identical in the earlier period of conch- 
ology. Since, however, the ancient name can only be retained 
for one of them, it must be reserved for that species (the for- 
mer, the Crenella discors of the British Mollusca, pl. 45, f. 5) 
with which the description more precisely accords. 
A New Zealand shell, the Crenella impacta of Hermann, had 
long usurped the Linnean name, but the authenticated Nor- 
wegian locality attributed to discors would alone suffice to inva- 
lidate its claims to identity. The C. marmorata of Forbes, 
the M. discors of the earlier writers upon British Testacea, is 
not present in the Linnean collection. 
Movtilus Hirundo. 
The name hirundo cannot be exclusively applied to any par- 
ticular species of Avicula, since all the members of that genus 
at that time known to science were grouped together in the 
synonymy attached to the species (?) in both editions of the 
‘Systema,’ and almost equally answer to the meagre descrip- 
tion. Among them are delineations of crocea, Tarentina, 
macroptera, and semisagitta, the two former of which are pre- 
sent (neither of them marked) in the hirundo box of the Lin- 
nean cabinet. Had the details of the ‘Museum Ulrice’ been 
less comprehensive, or its shorter synonymy purer (each of the 
four figures there cited represents a different Avicula) the name 
hirundo might still have been continued as a specific appella- 
tion. 
